Various types of flow rate sensors (or flow velocity sensors) which measure flow rates (or flow velocities) of various types of fluids, especially liquids, have heretofore been used, and so-called heat type (especially indirectly-heated type) flow rate sensors have been used for a reason that prices are easily reduced. As the indirectly-heated flow rate sensor, a sensor has been used in which a sensor chip obtained by stacking a thin-film heating element and a thin-film temperature detecting element via an insulating layer using a thin-film technique on a substrate is disposed in such a manner that heat can be transferred between the sensor chip and the fluid in a piping which is a fluid channel. When the heating element is energized, the temperature detecting element is heated, and values of electric properties such as electric resistances of the temperature detecting element are changed. The change of the electric resistance value (based on a temperature rise of the temperature detecting element) changes in accordance with the flow rate (flow velocity) of the fluid flowing in the piping. This is because a part of an amount of generated heat of the heating element is transferred into the fluid, the amount of the heat diffusing in the fluid changes in accordance with the flow rate (flow velocity) of the fluid, the amount of heat supplied into the temperature detecting element accordingly changes, and the electric resistance value of the temperature detecting element changes. The change of the electric resistance value of the temperature detecting element differs also at a temperature of the fluid. Therefore, a temperature detector for temperature compensation is incorporated in an electric circuit which measures the change of the electric resistance value of the temperature detecting element, and changes of flow rate measurement by the temperature of the fluid are reduced as much as possible.
This type of indirectly-heated flow rate sensor using the thin-film elements is described, for example, in JP(A)-11-118566. In the flow rate sensor, an electric circuit including a bridge circuit is used in order to obtain an electric output corresponding to the flow rate of the fluid.
Additionally, in recent years, importance of detection of leakage of the fluids from tanks or piping systems has increased. For example, when the oil leakages are generated from tanks of fuel oils such as gasoline, light oil, and kerosene, and a large amount of oil continuously leaks, problems such as fire breakout, environmental pollution, and resource loss are caused, and it is therefore extremely preferable to detect oil leakage generation in an initial stage. Therefore, oil leakage detection is sometimes required, for example, by a trace amount of 1 milliliter/h or less.
It is considered that the above-described indirectly-heated flow rate sensor is used in the detection of the oil leakage. However, in the flow rate sensor, the change of the output of the electric circuit with respect to a flow rate change is reduced in a region in which a flow rate value is a trace amount of 1 milliliter/h or less, there is a problem that an error of the flow rate measurement increases (i.e., a ratio of a distinguishable flow rate difference increases during measurement, and measurement sensitivity drops).
On the other hand, as the flow rate sensor, there is a two-constant-point temperature difference detecting system in which a fluid is heated by a heat source disposed in a specific position of a piping, temperature detecting elements are disposed at appropriate distances on upstream and downstream sides of a heat source position concerning fluid circulation in the piping, and a fluid flow rate is measured based on a detected temperature difference between upstream and downstream temperature detecting elements generated during the circulation of the fluid in the piping. However, in a case where the sensor is used in the detection of the oil leakage, when the flow rate value is, for example, 3 milliliters/h or more, the change of the output of the electric circuit with respect to the flow rate change is small, and therefore there is a problem that the error increases in a large flow rate value region (i.e., the ratio of the distinguishable flow rate difference increases during the measurement, and the measurement sensitivity drops).
Furthermore, the fuel oil tanks in a gas station and the like have heretofore been buried underground in most cases, and the piping which pumps out the fuel oil from the underground tank has also been buried underground. Micro cracks are generated in the piping by time degradation before long, and there is a very strong possibility that oil leakage is generated from the cracks. In this situation, surrounding environment pollution is caused, and enormous expenses are required for recovery. Therefore, the underground burial piping connected to the underground tank is periodically inspected for presence of oil leakage (or the crack in the piping, which is a cause for the leakage).
As a method which has heretofore been used for this piping inspection, there has been a method in which a gas such as fair or a liquid such as water is pressurized/injected into the piping in a sealed state of the piping, and the presence of a pressure drop after elapse of a predetermined time is detected. Conversely there has also been a method in which the tank is decompressed in a sealed state in the piping, and the presence of pressure increase after the elapse of the predetermined time is detected. However, in these methods, an operation for sealing all openings in the piping with putty or the like is required prior to the leakage inspection operation, an operation for extracting all the oil in the piping is required, and the operations are very troublesome. Additionally, when the openings are not completely sealed, the leakage detected in these methods does not necessarily reflect actual oil leakage based on the cracks in the piping, and it cannot be said that precision is high for labors of the inspection operation.
To quickly cope with the leakage of the liquid in the piping, it is essential that the leakage can be detected at an early stage at which the cracks in the piping and the like are small and the leakage is little, and therefore there has been a demand for the detection of a small amount of leakage.